1. Field of the Invention
The present invention relates to an image processing apparatus for processing image data in an image processing equipment such as a copying machine. The present invention relates to, in particular, determination as to whether the image is a monochrome or color image.
2. Description of the Related Art
Conventionally, an image forming apparatus for reading a monochrome or color image from an original on which the image is shown and for reproducing the image on a medium such as a printing paper. In the image forming apparatus of this type differs greatly in time required for processing, power consumption and the like, depending on whether or not an image to be reproduced is a monochrome or color image. For that reason, the apparatus switches operation modes depending on whether the image is a monochrome or a color image.
Meanwhile, there are cases where a person who is not familiar with the operation of office automation (or OA) equipment uses the image forming apparatus of this type. It is, therefore, desirable to automatically select an operation mode according to an image on an original. In view of this, the image forming apparatus incorporates an image processing apparatus to determine whether an image read is a monochrome or a color image and performs image forming operation in an operation mode based on the determination result.
The conventional image processing apparatus determines whether an image is a monochrome or color image as follows. It is determined whether every pixel included in an image is a monochrome or color pixel and the number of pixels determined as color pixels is counted. If the ratio of the number of color pixels thus counted to the number of all pixels included in the image is higher than a predetermined threshold, then it is determined that the image is a color image. If it is determined as a color image, image formation is conducted in a color image mode. If not, image formation is obviously conducted in a monochrome image mode.
That is, an image is not determined as a color image just because at least one color pixel is included in an image. A threshold for determination is set, for the following reasons.
First, there is sometimes a trace of a color on an inherent monochrome image, caused by a person's accidentally touching, for example, a color ball paint pen. In this case, it is preferable to deal with the image as a monochrome image as a whole even if color pixels are included therein more or less.
Second, some monochrome images are printed by superimposing the three primary colors. In that case, if registration accuracy is low, color pixels may appear on a boundary between black and white. It is inappropriate to determine such an image as a color image. Also, it is not a few possibilities that a monochrome pixel itself is erroneously determined as a color pixel at some level of accuracy to determine whether a pixel is a monochrome or a color pixel.
Third, there are cases where a color pixel is included in an image read from a monochrome original depending on the aberration of an optical system for reading images. It is also inappropriate to determine the image as a color image based on the above situation.
The conventional image processing apparatus as stated above basically determines an image based on the ratio of the number of color pixels to that of all of the color pixels in the image. Due to this, the following problems arise.
Even a color image might be determined as a monochrome image, depending on the original. This applies to an image including a signature written in red on a portion of a monochrome letter-based original and an image on which an underline is partially drawn with a color pen on a monochrome letter-based original. Even if a person who copied the image of this type recognizes it as a color image, the ratio of color pixels to all of the pixels is surprisingly low. The result is, the ratio may unlikely reach a threshold.
Nevertheless, it is possible to determine the image of that type as a color image by setting a low threshold. If so setting, however, it is more likely that an image to be dealt with as a monochrome image is determined as a color image, which does not eventually overcome the problem.